I would add:
The fastest way to move the origin is Opt+Shift (Alt+Shift on Windows, Linux, not sure
). This way is also a bit 'safer' since you have to be on a node for it to work. With the button method you can accidentally slip and end up close rather than on. To place the origin in empty space requires the button.
As for putting the origin back. There seem to be two types of users, those who move the origin a lot and keep the x&y button absolute (not pressed), and those who leave the origin alone and keep the x&y button relative (pressed). I am firmly in the first camp. By moving the origin, it's easy to, e.g., place a door 6" out of a corner. Yes, I know about the x3+ syntax, but I've never liked it, I prefer a visible, firm reference point, the origin. So I rarely put the origin back, I'm always moving it to the next place.
When you move the origin all the time, it becomes almost subconscious to check dims on the fly as Keith describes.
Another way to measure is to draw a line (or wall, anything linear). As you draw, the R box will give absolute length. For X&Y to be accurate with this method you need to be in relative mode (x&y button down). You don't need to actually finish the element, just start at one point, rest on the other, read the boxes, then cancel (esc). I use this when measuring to an angled element; the perpendicular cursor tells you when you are at the true minimum.
One more thing. Unless your working units and dimensioning units are set to the same tolerance, using a dim element to measure can give a misleading (rounded) value.
HTH,